COMPRESS2

where name is a symbol and alist is a 2-dimensional array named
name. See arrays for details. Logically speaking, this
function removes irrelevant pairs from alist, possibly shortening
it. The function returns a new array, alist', of the same name and
dimension as alist, that, under aref2, is everywhere equal to alist.
That is, (aref2 name alist' i j) is (aref2 name alist i j), for all
legal indices i and j. Alist' may be shorter than alist and the
non-irrelevant pairs may occur in a different order in alist' than
in alist.

Practically speaking, this function plays an important role in the
efficient implementation of aref2. In addition to creating the new
array, alist', compress2 makes that array the ``semantic value'' of
name and allocates a raw lisp array to name. For all legal indices,
i and j, that raw lisp array contains (aref2 name alist' i j) in
slot i,j. Thus, subsequent aref2 operations can be executed in
virtually constant time provided they are given name and the alist'
returned by the most recently executed compress2 or aset2 on name.
See arrays.